Amplifiers with low input noise and high input impedance are desired in certain applications, such as where the input signal is derived from a high impedance source with a low level signal output. Examples are instruments where an input signal is generated by an infrared sensor such as a bolometer or photovoltaic cell. In such cases, it has been customary to employ a lock-in amplifier comprised of a tuned pre-amplifier followed by a phase sensitive detector with a low-pass filter. Typically, great care has been required with the pre-amplifier design.
The use of bipolar transistors as the active component of the first stage of such an amplifier has not always proved satisfactory because the input impedance would be too low and/or because its current noise would be too high. Where a field effect transistor (FET) was substituted for the bipolar transistor in the first stage of the amplifier, the input impedance no longer presented a problem because the input impedance of the device was inherently high but other problems remained. The state-of-the-art in sensor amplifiers includes an amplifier with a FET first stage driving a second stage which is often an operational amplifier. However, to obtain a low noise characteristic from the first stage, a large drain to source current is required which necessitates the use of a small load resistor. This, in turn, results in a low first stage gain which allows a significant portion of noise from the second stage to effectively appear at the input to the first stage. This undesirable characteristic can be avoided by employing a low noise second stage operational amplifier; however, these devices are very expensive. The prior art, therefore, does not provide an inexpensive, low noise amplifier with a high input impedance characteristic.